A number of techniques are known for the protection of steel constructions from the effect of corrosion. One known technique is hot dipping galvanizing, wherein a thin layer of zinc is applied to the object surface. The applied zinc layer provides the object with a cathodic protection, i.e. in the case of corrosion zinc is relinquished and thus protects the underlying metal. The corrosion products of zinc will moreover fill up possible damage such as scratches and the like, whereby an additional protection is obtained.
The zinc can be deposited by electrochemical means onto the object, a process known as electrolytic galvanization. Alternatively, the zinc can be applied to the metal object by spraying zinc onto the surface of the object using spray guns (zinc-spraying), by having zinc diffuse in a drum (sherardizing) or by painting the zinc onto the object (referred to as zinc dust painting or cold-galvanizing). A further option for applying zinc to a metal is thermal galvanizing, wherein the object for treating is immersed in liquid zinc located in a zinc bath at temperatures between 445° C. and 465° C.
In thermal galvanizing the object for treating undergoes a pretreatment in which dirt, oil and fat residues are removed from the object surface. As pretreatment the object is then placed in a bath with a diluted hydrochloric acid solution and pickled therein so as to remove rust and mill scale. There then follows a “flux” treatment in which the object for treating is arranged in a flux bath with for instance zinc ammonium chloride so as to later obtain a good adhesion of the zinc to the steel. If the flux is first applied and then dried, this is known as dry galvanizing. In wet galvanizing, the flux is spread over the zinc bath surface and the steel is pulled therethrough. After the treatment there is formed on the steel surface an entity of zinc/iron alloy layers. After said pretreatment the object is immersed, in accordance with the known method, for some minutes in the zinc bath where the liquid zinc bonds to the steel, this over the entire surface thereof and therefore also on the inside of possible hollow structures in the object. During the immersion a number of (gamma, delta and eta layer) alloy layers are formed through reaction of zinc with metal, while a layer of pure zinc is formed when the object is taken out of the zinc bath.
A number of drawbacks are associated with the known methods. Firstly, the use of chemical baths such as hydrochloric acid baths as pretreatment of the steel has an environmental impact. In addition, the supply of hydrochloric acid and the discharge of (contaminated) hydrochloric acid entails high costs.
The known method further involves a number of labour-intensive and relatively costly steps, such as the arranging of the steel in degreasing baths, pickling baths and possible dezincification baths in the case of reconditioning of steel once galvanized in the past. The hydrochloric acid only removes the mill scale from the object and further impurities remain present on the object surface. Additional processing steps are hereby necessary.
A further drawback of the known method and device is that the use of hydrochloric acid results in brittleness of the treated metal. Subsequent galvanizing of the brittle metal will therefore produce a less smooth surface, which adversely affects the appearance of the galvanized product.
Known from the American document U.S. Pat. No. 5,666,714 is a method for galvanizing steel components. The components are first shot-blasted and are then formed and/or welded into an object. The thus formed and/or welded object undergoes, among others, a flux treatment and a galvanizing treatment. A drawback of the known method is that prefabricated and/or used objects first have to be taken apart into their individual components to enable the start of pretreatment of the components.